1. Field of the Invention
The present invention relates to a cordless telephone system which comprises a connection unit connected to a wire telephone circuit and a radio telephone set connected through a radio communication circuit to the connection unit and, more particularly, to a cordless telephone system which effectively utilizes a radio communiction circuit between a connection unit and a radio telephone set.
2. Description of the Related Art
A cordless telephone system comprises a connection unit connected to a wire telephone circuit and a radio telephone set connected through a radio communication circuit to the connection unit. Such a cordless telephone system, when it is desired to make a call from the radio telephone set, is operated according to a flowchart as shown in FIG. 6(a) and (b). More specifically, when the radio telephone handset is first picked up, i.e., put in its off-hook state, the radio telephone set sets the radio communication circuit at a control channel (step 201) and subsequently determines whether or not the control channel is idle (step 202). If the control channel is not idle, then the telephone set waits for a predetermined constant time (step 203) and then generates a busy tone (step 204) to return to its await mode. If the control channel is idle, then the radio telephone set turns on its transmitter (step 206) and transmits a calling signal to the connection unit (step 207).
The connection unit, when receiving the calling signal (step 101), transmits an answer signal containing information indicative of an idle communication or speech channel in response to the calling signal (step 102) and then sets the radio communication circuit at the aforementioned idle communication channel (step 103).
The radio telephone set, when receiving the aforementioned answer signal (step 208), sets the radio communication circuit at the idle speech channel specified by the answer signal (step 209). And the radio telephone set determines whether or not the speech channel is idle (step 210). If the speech channel is idle, then the telephone set turns on its transmitter to transmit a channel switching completion signal to the connection unit (step 211).
The connection unit, when receiving the channel switching completion signal (step 104), turns on a transmitter of the unit (step 105) and determines whether or not interference waves are present in the received electric field (step 106).
The radio telephone set, on the other hand, determines whether or not the transmitter of the connection unit has been turned on (step 212) and further determines the presence or absence of any interference waves in the received electric field (step 213). The absence of the interference waves causes the radio telephone set to transmit an interference-wave detection completion signal to the radio telephone set (214).
The connection unit, when receiving the interference-wave detection completion signal (step 107), transmits a voice-circuit ON signal to the radio telephone set (step 108) and thereafter enters into its communication mode (step 109).
The radio telephone set, when receiving the voice-circuit ON signal (step 215), enters into its communication mode (step 216).
In the communication mode of the radio telephone set, the telephone set detects whether or not the received electric field of the speech channel is not smaller than E2 dB.mu.V (for example, 20 dB.mu.V) (step 217) and starts measuring an elapsed time after the moment the received electric field has reached less than E2 dB.mu.V. When the measured time reaches a predetermined time T1, the telephone set generates an alarm sound (step 219). When a time (T1 + T2) further elapses (step 230), the telephone set transmits a speech end signal to the connection unit through the speech channel (step 234) and cuts off the radio communication circuit of the speech channel (step 235). When the received electric field is at a level of E2 dB.mu.V or higher or when the electric field is lower than E2 dB.mu.V and the time T1 does not expire yet, the telephone set turns off the alarm sound (step 220). Under such conditions, a dialing operation (step 231) causes the telephone set to send a dial signal to the connection unit (step 232), and an on-hook operation (step 233) causes the telephone set to send a speech end signal thereto (step 234).
On the other hand, the connection unit detects whether or not the received electric field of the speech channel is smaller than E1 dB.mu.V (for example, 0 dV.mu.V) (step 110) and starts measuring an elapsed time after the moment the electric field becomes smaller than E1 dB.mu.V. When the measured elapsed time reaches a predetermined time T3 (step 111), the connection unit disconnects the radio communication circuit of the speech channel (step 115). The connection unti with the received electric field at a level of E1 dB.mu.V or higher, when receiving the dial signal (step 112), outputs the dial tone onto the wire telephone circuit (step 113).
In this manner, the radio telephone set disconnects the radio communication circuit of the speech channel when the received electric field is at a level of less than E2 dB.mu.V, while the connection unit disconnects the radio communication circuit of the speech channel when the electric field is at a level of less than E1 dB.mu.V. For this reason, the radio telephone set is normally disconnected faster than the connection unit. However, in the case where a difference between the transmit and receive frequencies of the speech channel is 100 MHz or more, when an antenna is incorporated in the radio telephone set, the received electric field of the connection unit sometimes reaches a level smaller than E2 dB.mu.V faster than the received electric field of the radio telephone set reaches a level smaller than E1 dB.mu.V, because the orientation or direction of such incorporated antenna causes variations of the propagation condition of the received/transmitted radio waves and of the antenna gain, and so on. If such a phenomenon takes place, then the connection unit first disconnects the radio communication circuit and then after the radio telephone set has measured the lapse time (T1 + T2), the telephone set disconnects the radio communication circuit. This causes the radio telephone set to transmit unnecessary waves for the time (T1 + T2).
Explanation will next be made as to the case where an application of a ringing tone from the wire telephone circuit to the connection unit causes the radio telephone set to receive a call. In this case, the connection unit and radio telephone set are operated based on a flowchart as shown in FIG. 7(a) and (b). More in detail, the connection unit, when receiving a ringing tone from the wire telephone circuit (step 301), first sets the radio communication circuit at the control channel (step 302). If the connection unit determines the control channel is idle (step 303), then it transmits to the radio telephone set a call indicator signal containing information indicative of an idle speech channel (step 304).
The radio telephone set, when receiving the call indicator signal (step 401), transmits an answer signal in response to the call indicator signal to the connection unit (step 402) and thereafter sets the radio communication circuit at the speech channel specified by the call indicator signal (step 403). The telephone set then determines whether or not the speech channel is idle (step 404). If idle, then the telephone set transmits a channel switching completion signal to the connection unit (step 405).
On the other hand, the connection unit, when receiving the call-indicator answer signal from the radio telephone set (step 305), sets the radio telephone circuit at this speech channel (step 306). Thereafter, the reception of the channel switching completion signal from the radio telephone set (step 307) causes the telephone to turn on the transmitter (step 308) and to determine the presence or absence of any interference waves in the received electric field (step 309).
The radio telephone set determines whether or not the transmitter of the connection unit is turned on on the basis of the received electric field (step 406) and when determining the absence of any interference waves in the received field (step 407), transmits an interference-wave detection completion signal to the connection unit (step 408).
The reception of the interference-wave detection completion signal (step 310) causes the connection unit to judge whether or not the ringing tone is still being sent from the wire telephone circuit (step 311). If it is still being sent, then the connection unit transmits a bell ringing signal to the radio telephone set (step 312).
The reception of the bell ringing signal (step 409) causes the radio telephone set to ring the bell to inform the operator of the receiving call (step 410), whereas no reception of the bell ringing signal causes the telephone not to ring the bell (step 411). When not ringing the bell, the telephone measures a predetermined time T3 and after passage of the time T3 (step 412), the telphone enters into the await mode (step 413). When the telephone rings the bell or when the time T3 does not elapse, it is determined whether or not the received electric field is not smaller than E2 dB.mu.V (step 414). If the electric field is smaller than E2 dB.mu.V, then the controller measures a predetermined time T4. The passage of the time T4 (step 415) causes the telephone to enter into the await mode (step 413). When the time T4 does not expire or when the received field is equal to or larger than E2 dB.mu.V, the controller judges whether or not the handset of the telephone has been picked up, i.e., the telephone is in an off-hook state (step 416). If it is in the off-hook state then the telephone transmits an off-hook signal (step 417). Thereafter, the reception of a voice-circuit ON signal (which will be explained later) from the connection unit (step 418) causes the telephone to be put in the communication mode (step 419).
The connection unit, when determining the absence of the ringing tone at the step 311, judges at a step 313 whether or not the received electric field is not smaller than E1 dB.mu.V. When the received field is smaller than E1 dB.mu.V, the connection unit measures a time T5 to judge if the time T5 has expired (step 314). At this point, if the time T5 does not expire or if the connection unit judges the received field is not smaller than E1 dB.mu.V at the step 313, then the connection unit judges whether or not it has received the off-hook signal (step 316). The reception of the off-hook signal causes the connection unit to transmit the voice-circuit ON signal (step 317) and to enter into the communication mode (step 318). The expiration of the time T5 at the step 314 causes the connection unit to enter into the await mode (step 315).
In this way, the radio telephone set detects whether or not the received electric field of the speech channel is not smaller than, for example, E2 dB.mu.V (step 414) and starts measuring a lapse time after the moment the field has reached a level smaller than E2 dB.mu.V. When the measured time reaches the predetermined time T4 (step 415), the telephone is put in the await mode (step 413).
On the other hand, the connection unit detects whether or not the received electric field is not smaller than, for example, E1 dB.mu.V (step 313) and starts measuring a lapse time after the moment the field has reached a level smaller than E1 dB.mu.V. When the measured laspe time reaches the predetermined time T5 (step 314), the connection unit is put in the await mode (step 315).
However, even in the case of the presence of a received call, if the received electric field becomes smaller than E1 dB.mu.V faster than the received electric field of the radio telephone set becomes smaller than E2 dB.mu.V, then the connection unit first disconnects the radio communication circuit and then, after having measured the time T4, the radio telephone set disconnects the radio communication circuit. This results, as in the calling mode, in that the radio telephone transmits unnecessary waves for the time T4.
Explanation will next be made as to how a cordless telephone system comprising a connection unit attached with a wire telephone set gives a call and receives a call. In the case where the system is to be put in the calling mode, the connection unit and radio telephone set are operated according to the calling operation of such a flowchart as shown in FIG. 6(a) and (b) to establish a radio communication circuit for a speech channel. In this case, if the wire telephone set of the connection unit occupies a wire telephone circuit, then the system must inform a user of the radio telephone set that the wire telephone circuit is already occupied. To this end, it is considered that the connection unit transmits a busy tone to the radio telephone set. However, this method is not efficient from the viewpoint of the effective utilization of radio waves, since the radio communication circuit of the speech channel is used during the transmission of the busy tone.
In the case where the system is to receive a call, if the wire telephone set answers the call during ringing of the radio telephone set, then the radio telephone cannot receive the bell ringing signal after passage of the time T3 as shown by the flowchart in FIG. 7(b) (step 412) or the radio telephone must disconnect the radio communiction circuit in response to the reduction of the received electric field (step 414) to return to the await mode, in which case the radio telephone set transmits unnecessary radio waves as in the above case, though, for a short time.
As has been explained in the foregoing, the prior art cordless telephone systems are disadvantageous in this, even when the radio communication circuit between the connection unit and radio telephone set is not used for communication, the radio communiation circuit is not disconnected and unnecessarily used.
It is a primary object of the present invention, therefore, to provide a cordless telephone system which prevents the unnecessary use of a radio communication circuit when not being utilized for communication.